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The induction of cytogenetic damage (micronuclei) in mouse fetal blood was studied with four selected mutagens: cyclophosphamide, procarbazine, trenimon, and mitomycin-C. For comparison the standard micronucleus test on maternal bone marrow was also performed. In contrast to the results obtained from maternal bone marrow the changes in the cellular composition in fetal blood were only slight after treatment with mutagens. A significant and dose-dependent increase in the incidence of micronucleated fetal blood cells was found with all four mutagens. The inducibility of micronuclei by indirect mutagens was particularly interesting. The three mutagens other than mitomycin-C induced a higher frequency of micronucleated polychromatic erythrocytes in fetal blood cells than in maternal bone marrow. The results indicate that this modified micronucleus test is well suited and useful for mutagenicity screening of environmental chemicals and especially for assessment of risks to the fetus when pregnant females are exposed to environmental chemicals.